In on-line manufacturing processes, for ink printed articles such as beverage cans, there is a problem of checking the colour consistency of the ink printing. In particular, there are problems in colour inspection, in capturing a colour image of a moving object on a production line; defining the areas of an article in which the colour is to be monitored, given the complex printed designs on the articles; and transforming a colour image signal into a uniform colour space and efficiently processing the colour image signal in the colour space.